1. Field of the Invention
The present invention relates to an improvement in a solid state color imaging apparatus.
The present invention especially concerns a solid state color imaging apparatus comprising a solid state imaging sensor having a plurality of photoelectric elements disposed in a two-dimensional pattern and is a specially patterned color filter intended to attain high-horizontal resolution, high S/N ratio and simple signal processing.
2. Description of the Prior Art
A solid state image sensor is, as is well known, an apparatus for converting an objective image to an electric signal. A single chip solid state color camera is an apparatus comprising a solid state imaging sensor and color filter with elements of different colors in front of the solid state imaging sensor to obtain an objective image on the surface of the solid state image sensor, which produces a spatially modulated chrominance signal and luminance signal to enable the syntheses of a composite color signal.
On the other hand, the solid state image sensor is a kind of a large scale integrated circuit (LSI), and therefore in order to obtain a high production yield, the degree of the integration of the device should be preferably low. Therefore, the number of the photoelectric elements, disposed in a two dimensional pattern on a substrate, namely, the number of picture elements, should be as small as possible under a condition for attaining a required resolution of the image. Accordingly, the way of producing a chrominance signal should be that which does not lower the theoretical value of the resolution attainable for the number of picture elements. Many proposals have been made for the modes of the pattern combination between the picture elements and the color filter thereof, and the main stream thereof are those using a mosaic color filter.
The problems in the conventional type solid state color imaging apparatus are elucidated referring to FIGS. 1 to 3. FIG. 1 shows a fundamental type apparatus which uses a color filter having filter pattern of vertical color stripes comprising repetition of the three primary colors in a horizontal direction. The apparatus has a solid state image sensor having a number of photoelectric elements, i.e., picture elements 101 disposed forming vertical rows and horizontal lines, and a color filter having vertical color stripes of green filter stripes 102, red filter stripes 103 and blue filter stripes 104 disposed repeatedly in turn. In the signal produced based on sampling by the picture elements of the objective image which is spatially modulated by the color filter stripes, the chrominance signals for colors of the color filter are modulated by respective color filter stripes. In this example, since the spatial repetition period of the filter is for three picture elements length, the carrier frequency of the chrominance signal is one third (1/3) of the sampling frequency f.sub.S by the picture elements. In this case, the band-width of the signal of the objective image sampled by the picture elements is a half (1/2) of the sampling frequency of the picture elements. In order to avert the undesirable inclusion of the chrominance signal carrier frequency f.sub.C in the luminance signal band, the frequency range usable for the luminance signal band has to be limited to under 1/3f.sub.S (one third of the sampling frequency), resulting in a disadvantage of lowering of the horizontal resolution.
FIG. 2 shows another conventional example wherein green filter elements 202 are disposed in a staggered way with respect to the photoelectric picture elements 201 for other picture elements in the n-th line (for instance, odd number lines) R (red) filters 203 are disposed and for other picture elements in the (n+1)-th lines (for instance, even order lines), B (blue) filters 204 are disposed. In this conventional example, by utilizing a green signal for the luminance signal as an approximation and by utilizing vertical correlations of signals, the luminance signal is obtained with a high resolution without the problem as in the conventional example of FIG. 1 where the band width of the luminance signal was limited by the carrier of chrominance signal. However, in this example because the vertical correlation is used, a distortion is produced for an object without vertical correlation thereby lowering picture quality. Also, since the red component and green component signals are generated from the picture elements of the n-th lines and the green component and blue component signals are generated from the (n+1)-th lines, the sum of the signal components differs between even lines and odd lines, thereby vertical non-uniformity or undesirable horizontal stripes is produced when the luminance signal is separated by using only a low-pass filter. Accordingly, the circuit for separating the luminance signal becomes complicated.
Further, there are other conventional examples as shown in FIG. 3. In this example, for a picture element of an n-th line (e.g., odd number line), 2/3 of the area is covered by a red filter 303 and the remaining 1/3 area is covered by green filter 305 and in the next picture element in the same horizontal direction, 2/3 of the area is covered by blue filter 304 and the remaining 1/3 area is by a green filter, and the same pair repeats in the same order in that line. On the other hand, in the neighboring line of the picture element, i.e. (n+1)-th line (e.g. even number lines), in a picture element which is under a former one of the n-th line, 2/3 of the area is covered by a blue filter and 1/3 of the area is covered by a red filter, and in next picture element in the same horizontal line, 2/3 of the area is covered by G filter and the remaining 1/3 of the area by a red filter, and such pairs repeat in the horizontal direction in (n+1)-th lines. In this conventional example of FIG. 3(a), luminance signal S.sub.Yn for the n-th line and luminance signal S.sub.Y(n+1) for the (n+1)-th line are given by the following equations (1) and (2): EQU S.sub.Yn =(2/3R+1/3G)+(2/3B+1/3G)=2/3(R+G+B) (1) EQU S.sub.Y(n+1) =(2/3B+1/3R)+(2/3G+1/3R)=2/3(R+G+B) (2)
In this example, since the components of the luminance signals or n-th lines and (n+1)-th lines have the same contents, the luminance signal can be easily obtained only by taking out through a low-pass filter, without a complicated sampling process as shown in the example of FIG. 2.
However, this conventional example has the difficulty of making the filter so as to be registered with the picture elements in a manner to divide the picture element into 2/3 areas and 1/3 areas. Furthermore, in this conventional example, the chrominance signals which are spatially modulated for n-th lines and (n+1)-th lines are given by the following equations (3) and (4), respectively: EQU S.sub.Cn =K.sub.1 (2/3R+1/3G)-(2/3B+1/3G) sin 2.pi.f.sub.C t+=K.sub.1 2/3(R-B) sin 2.pi.f.sub.C t+ (3), EQU S.sub.C(n+1) =K.sub.1 {(2/3B+1/3R)-(2/3G+1/3R)} sin 2.pi.f.sub.C t+=K.sub.1 2/3(B-G) sin 2.pi.f.sub.C t+ (4),
where K.sub.1 is a constant.
As shown in the equations (3) and (4), the modulated chrominance signals have components of primary color signals in the amounts of 2/3. Accordingly, the S/N ratio is low. As shown in equations (1) and (2), the luminance signals become in the amount of 2/3 of the primary color signals R, G and B. Accordingly the S/N ratios of the luminance signals also are not sufficiantly high.